This invention relates generally to a detonation indication system, and more specifically to an onboard aircraft engine detonation indication system, which senses detonation through the use of combustion pressure sensing force transducers located under the spark plugs of each cylinder.
The threat of imposing unleaded aviation gasoline regulations is causing great concern in the aviation industry, since most of the larger displacement piston engines of the existing fleet require 100 octane fuel. There is no economically acceptable alternate method of producing 100 octane fuel in the absence of tetra ethyl lead. The proposed changes in fuels will force a reduction of the octane levels which will greatly affect the performance of these large piston aircraft engines in their present configurations. The power to weight performance of the aircraft is, of course, crucial and the engines must operate at their peak performance levels with these anticipated octane reduction changes. The key means to restore power losses is the development of appropriate engine modifications and/or tuning changes. However, a tradeoff in developing engine modifications and tuning changes is the risk of detonation (knock) which can be destructive to an engine.
It is anticipated that the lesser octane unleaded fuels of the future may require recertification of a significant number of aircraft with potential revisions as to lifting capacity, load and engine performance, depending on the combined impact of fuel knock characteristics and engine modifications or tuning changes adopted. The present system was designed to perform ground and flight evaluations of new candidate unleaded aviation gasolines, and to facilitate the extensive recertification effort that lies ahead. Current methods of monitoring knock lack the precision required to evaluate new fuels, and place difficult monitoring and management demands on test pilots in the air. A detonation indication system with simplified management characteristics is needed, which can be quickly read without adjustments to determine engine knock for each cylinder of each engine. The instrument needs to detect not only intensity of the knock events but provide an indication of events frequency as well.
One of the first aircraft engine analyzers was the Sperry system which is set forth in U.S. Pat. No. 2,518,427. The Sperry system which incorporates an oscilloscope display, analyzed various other malfunctions, besides detonation such as defective valve train components, spark plugs, magnetos, incorrect ignition timing, mixture and various other engine irregularities. The Sperry system has been accepted as the standard by the regulatory agencies in the certification of aircraft. However, it is large, cumbersome and requires a substantial amount of accessory equipment which renders it difficult to use in flight for general aviation aircraft, and non-usable on the smaller members of this fleet.
The Sperry system also requires substantial adjustment before it can be used, along with a technician to perform functional mode selections, and to view and adjust the oscilloscope which certainly could not be safely done by the test pilot alone.
The AVCO Detonation Analyzer System was developed in more recent times. However, it also sensed vibration as in the Sperry system and presented the data on the screen of an oscilloscope. The AVCO system required a vibration pickup mounted on each cylinder and an engine mounted crankshaft encoder for referencing TDC (top dead center of each cylinder). The AVCO system demands a skilled operator to perform adjustments and considerable time of engine operation under knock conditions in the air, since the interpretation criteria is based on a count of knock events (light flashes) per minute.
Other modern engine sensors in the non-aviation field have been developed for various automotive applications. U.S. Pat. No. 4,450,811 belonging to the NGK Spark Plug Co., Ltd. is an example. The NGK controller is not a system to analyze the performance of an engine but rather a system for operating a fuel injected automotive engine which controlled the ignition timing and mixture to operate the fuel system of the vehicle. This system senses pressure in the cylinders through the use of piezoelectric washers on the head bolts. The NGK system requires measuring the crank angle (crankshaft encoder), which the present invention does not and its purpose is somewhat different in that when it senses knock, it automatically retards the timing.
Similar piezoelectric washers, as disclosed in the NGK patent above, have been used under spark plugs to sense knock in some Nissan automobiles for the same purpose as in the NGK system.
There are automotive engine analyzers on the market today such as the AVL Indiskop and Norland systems which are stationary units used in laboratories for engine development and diagnostic purposes. These systems provide a visual indication of knock conditions by means of cylinder pressure traces displayed on a screen, along with numerous other engine parameters not embodied in this invention.